This application is based on Japanese Patent Application No. 2002-113519 filed on Apr. 16, 2002 the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an air conditioner for motor vehicles. The present invention relates to an air conditioner that is capable of keeping comfortable feeling at a fuel cut control of a vehicle engine.
2. Description of Related Art
In a conventional motor vehicle, when the amount of accelerator operation is zero during deceleration, so-called fuel cut control is exercised. That is, the supply of fuel to the vehicle engine is cut off for the purpose of improving fuel economy of the vehicle engine. The engine is an internal combustion engine. The accelerator operation is usually detected as a depression degree of the accelerator pedal. In this case, in order to prevent engine stall, a fuel cut canceling speed is set at a lower-limit engine speed at which the vehicle engine will not stall even if the fuel supply is cut off. The fuel supply is resumed when the engine speed lowers below the predetermined canceling speed.
In a motor vehicle mounted with an air conditioner, the air-conditioner compressor is driven by the vehicle engine. When the compressor is operating, therefore, the vehicle engine drive load becomes greater by the amount of a compressor load torque than when the compressor is not operational. Therefore, the fuel cut canceling speed NEa during compressor operation is set higher by the predetermined speed than the fuel cut canceling speed NEb during a stop of compressor operation as shown in FIG. 4. Therefore, if the compressor is operational, the fuel supply is resumed at a relatively higher engine speed and the fuel supply is resumed relatively earlier. Therefore, when the compressor is driven, the fuel cut time period will become shorter than when the compressor is not driven.
The inventors have studied an experimental system. In this system, in order to extend the fuel cut time period, the canceling speed is decreased as low as the canceling speed when the compressor is not driven. For this purpose, a variable displacement compressor capable of changing the displacement was adopted as an air-conditioner compressor. In addition, the vehicle engine driving load is reduced by reducing the compressor displacement to a small value during vehicle deceleration.
Simply reducing the displacement, however, will present such a problem that, during the summer season when the vehicle compartment is cooled, lowering the cooling capacity of the evaporator raises the temperature of the cold air to be supplied into the vehicle compartment. This will deteriorate a cooling sensation.
JP-A-6-115346 discloses a system that reduces the compressor displacement in order to prevent a vehicle engine stall at a sudden vehicle deceleration. Even this conventional technology also similarly deteriorates the cooling sensation by the lowered cooling capacity of the evaporator.
It is an object of the present invention to provide an air conditioner for motor vehicles that is capable of keeping comfortable condition even if the displacement of the compressor is reduced in response to deceleration.
According to a first aspect of the present invention, an air conditioner for motor vehicles comprises a refrigeration cycle and a controller. The refrigeration cycle has a variable displacement type compressor and an evaporator for cooling air supplied to a vehicle compartment. The compressor is driven by a vehicle engine that is subjected to a fuel cut control in vehicle deceleration. The controller controls at least the displacement of the compressor. The controller controls the displacement both when the fuel cut control is performed and is not performed. When the vehicle engine is supplied with fuel, that is, the fuel cut control is not performed, the controller controls the displacement on the basis of a cooling degree of the evaporator. When the fuel cut control is performed, the controller performs a forced reduction control of the displacement. In the forced displacement control, the controller reduces the displacement to a predetermined small value. Further, the controller sets a time period for the forced reduction control such that the time period is shortened as a thermal load for cooling is increased.
According to the present invention, a time period for the fuel cut control can be extended by the forced reduction control. Further, the time period for the forced reduction control is shortened as the thermal load for cooling is increased. For example, the time period is shortened as an air temperature to be cooled is increased. Therefore, the temperature rise of the air supplied to the vehicle compartment is restrained by shortening the time period of the forced reduction control of the displacement. Thus it is possible to keep comfortable cooling feeling and to restrain the deterioration of cooling sensation.
The cooling degree of the evaporator may be represented by a temperature or pressure relating to the evaporator such as an air temperature cooled by the evaporator, and an evaporator surface temperature.
The controller may perform a cold storage control when the fuel cut control is performed. The cold storage control may be performed just before the forced reduction control, and be performed by increasing the displacement insomuch that the cooling degree of the evaporator is kept higher than that achieved when the vehicle engine is supplied with fuel. For example, the cooling degree of the evaporator may be controlled higher than a normal degree that is capable of keeping a heat exchanging efficiency high. For example, the temperature of the evaporator in the cold storage control is controlled lower than that in a normal control.
The controller may gradually increase the displacement from a value achieved by the forced reduction control to a value achieved when the vehicle engine is supplied with fuel in response to a finishing of the fuel cut control. It is possible to prevent abrupt change of the air temperature supplied to the vehicle compartment.
The thermal load may be indicated by at least one of an outside air temperature and a volume of air to be supplied to the vehicle compartment. In this case, the time period may be shortened as the outside air temperature is increased or as the volume of air is increased.
The thermal load may be represented by a cooling degree of the evaporator measured actually. In this case, the time period may be shortened by completing the forced reduction control when the cooling degree reaches to a predetermined degree.
The controller may include means for controlling the displacement on the basis of the cooling degree, means for performing the forced reduction control, and means for regulating the time period for the forced reduction control.
The controller may control the displacement to a predetermined large value when information indicative of an engine speed indicates that the engine speed is decreased within a high speed range toward a predetermined engine speed from beginning of the fuel cut control. The controller begins the forced reduction control when the engine speed is decreased in a low speed range below a predetermined level.
The time period may be set by a displacement reduction regulating time. The displacement reduction regulating time may be defined in accordance with both the outside temperature and the volume of air. The displacement reduction regulating time is shortened as the outside temperature is increased when the outside temperature is in a predetermined high temperature range higher than a predetermined first temperature, e.g., 25xc2x0 C. The displacement reduction regulating time is shortened as the outside temperature is decreased when the outside temperature is in a predetermined low temperature range lower than a predetermined second temperature, e.g., 17xc2x0 C.
The displacement reduction regulating time may be set at a maximum period at a range located between the predetermined first temperature, e.g., 25xc2x0 C., and the predetermined second temperature, e.g., 17xc2x0 C.
The displacement reduction regulating time may be shortened as the volume of air is increased.